?– A provider bridge can implement standardized rate policing.
384 Chapter 13
?– A provider bridge has VID translation capability at each port.
?– A provider bridge has a ???C-tagged interface??? that can deal with the customers??™
Q-tags in interesting ways.
We will examine these modifications one at a time, in the context of the problems
they solve.
S-VLAN Tags The existing 802.1Q tag is now called a C-tag, or customer tag. Looking
back at Figure 13.3c, you can see that tag has an Ethertype (0 ?— 8100), a 3-bit priority
field, a 1-bit Canonical Format Indicator (CFI) field, and a 12-bit VLAN ID (VID). As
mentioned previously, a VID of 0 indicates ???unknown VID,??? so the VID is determined
by the bridge??™s configuration, and that VID 4095 (0 ?— FFF) is illegal. The priority field
has the same meaning as in the C-tag.
The CFI was defined for use in mixed token ring (IEEE 802.5) and Ethernet environments.
If this bit is set, it indicates that a variable-length routing information field
follows the C-tag and precedes the type/length field. This field has never found widespread
use and thus was available for redefinition in the S-tag.
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